Solving Open String Field Theory with Special Projectors

Schnabl recently found an analytic expression for the string field tachyon condensate using a gauge condition adapted to the conformal frame of the sliver projector. We propose that this construction is more general. The sliver is an example of a special projector, a projector such that the Virasoro operator \L_0 and its BPZ adjoint \L*_0 obey the algebra [\L_0, \L*_0] = s (\L_0 + \L*_0), with s a positive real constant. All special projectors provide abelian subalgebras of string fields, closed under both the *-product and the action of \L_0. This structure guarantees exact solvability of a ghost number zero string field equation. We recast this infinite recursive set of equations as an ordinary differential equation that is easily solved. The classification of special projectors is reduced to a version of the Riemann-Hilbert problem, with piecewise constant data on the boundary of a disk.